The present invention relates to a lithium battery, and more particularly, to a lithium battery using a lithium iron oxide as an electrode active material.
With the recent development of portable electrical equipments such as personal computers and handy phones, there is an increasing demand for batteries as the power source. Especially, a lithium battery using lithium which has a small atomic weight but a large ionization energy, produces a high energy density, and various studies have been made on lithium batteries.
As a cathode active material which produces a high electromotive force and increases the energy density in a battery, those producing a voltage of 4 V such as Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2 are extensively investigated.
However, since a battery using a compound containing cobalt or nickel such as Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2 costs too much, and the production of both cobalt and nickel is comparatively low, these compounds cannot be said to be optimal as a material of a battery for practical use. It is, therefore, expected to solve this problem by using an iron compound obtained by replacing Co and Ni in Li.sub.x CoO.sub.2 or Li.sub.x NiO.sub.2, respectively, with another transition metal element, in particular Fe, which is cheap and which is found in abundance.
Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2 which have excellent properties as an electrode active material have a layered rock salt-type (.alpha.-NaFeO.sub.2) crystal structure. As a compound having a stratified layered rock salt-type crystal structure other than Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2, Li.sub.x VO.sub.2 and Li.sub.x CrO.sub.2 are only known, and no lithium iron oxide having a similar crystal structure is known.
A lithium iron oxide obtained by, what is called a high-temperature synthesizing process, by calcining mixed particles of an iron oxide and a lithium compound at about 800.degree. C., has a disordered tetragonal rock salt-type crystal structure. On the other hand, a lithium iron oxide obtained by, what is called a low-temperature synthesizing process, by calcining mixed particles of an iron oxide and a lithium compound at about 400 to 500.degree. C., has an ordered tetragonal rock salt-type crystal structure. However no lithium battery produced by using such a lithium iron oxide as an electrode active material had a sufficient performance for practical use.
To solve the above-mentioned problems, a lithium battery using a lithium iron oxide: Li.sub.x FeO.sub.2 (0&lt;x&lt;2) having a corrugated layer structure similar to the crystal structure of the known Li.sub.x MnO.sub.2 (0&lt;x&lt;2) was proposed.
The lithium iron oxide: Li.sub.x FeO.sub.2 (0&lt;x&lt;2) having a corrugated layer structure is obtained by the ion-exchange reaction between protons in lepidocrocite (.gamma.-FeOOH) and lithium ions. Since lepidocrocite has a crystal structure containing protons between the corrugated layers, in the case where lepidocrocite is heated together with a lithium compound, an ion exchange reaction occurs, and simultaneously with the removal of the protons, lithium ions are introduced between the corrugated layers, so that a lithium iron oxide having a corrugated layer structure is obtained. When a lithium battery is produced by using such a lithium iron oxide as an electrode active material, the lithium ions between the FeO.sub.2 layers are electrochemically introduced into and removed from the lithium ion cites between the layers during the operation of the battery.
This lithium battery, however, has the following problems. It is necessary to carry out the ion-exchange reaction at a temperature as low as not higher than 350.degree. C., because when the ion-exchange reaction is carried out at a high temperature, .alpha.-LiFeO.sub.2 as a high-temperature stable phase is produced. Since the ion-exchange reaction is carried out at such a low temperature, the crystallinity of the lithium compound obtained is low, and the crystal structure tends to become unstable.
When lithium ions are repetitively electrochemically introduced into and removed from the ion cites of the lithium iron oxide having such an unstable crystal structure along with the charge and discharge of the battery, the crystal structures around the iron cites are likely to change, and as a result, the ion cites where the lithium ions can exist or the lithium ion conduction path in the crystals tends to disappear. Consequently, with the repetition of charge and discharge of the battery, a reduction in the capacity of the battery or an increase in the internal impedance tends to be caused.
The lithium iron oxide having a corrugated layer structure have a low electron conductivity and the diffusion of the lithium ions between the layers is slow. As a result, in a lithium battery using the lithium iron oxide, the electrode reaction rate is low and the electric current operated by the battery become small.
In addition, the crystal structure of the lithium iron oxide changes with the passage of time and lepidocrocite is apt to be produced. The change in the crystal structure lowers the activity as the electrode active material, and a reduction in the capacity of the battery or an increase in the internal impedance is apt to be caused, so that it is difficult to keep the characteristics of the lithium battery stable.
As a result of studies undertaken by the present inventors so as to solve these problems, it has been found that a lithium battery (lithium cell) using a lithium iron oxide containing a specific amount of at least one element selected from the group consisting of cobalt, nickel, manganese and aluminum, and having a corrugated layer crystal structure as an electrode active material for at least either of the electrodes, preferably a cathode active material, has an excellent cycle life, a large current drain operated and has stable characteristics of the battery. The present invention has been achieved on the basis of this finding.